Tijuana River mouth, Imperial Beach
This winter's El Niño wasn't much as weather goes, but its ocean-water effects — which closed the Tijuana River mouth multiple times since March — left the river estuary vulnerable to catastrophe, according to scientist Jeff Crooks.
"Hundreds of leopard sharks died — sharks aren't great at dealing with low oxygen…. Purple varnish clams and bubble snails died," Crooks said.
Flooding pushed endangered clapper rails — a shy, leggy bird with an orange bill — out of the center of the estuary to dry spots along Imperial Beach's Seacoast Drive, he said.
"You could do a clapper rail survey from your car driving around."
The flooding also hit Imperial Beach, sending water into low-lying areas including Seacoast Drive. No one was prepared for the rain and the sewage coupled with the river mouth closing, but they will be next time, Crooks said.
"Fish & Wildlife is working with the Army Corps of Engineers to have a standing permit to open the river mouth. They moved very fast on the emergency permit, and we learned that we should be prepared to do this again."
The river mouth closed on March 31st, when wave energy and sand from the SANDAG 2012 replenishment project filled it in. The sea-life kill began eight days later, while the City of Imperial Beach and U.S. Fish & Wildlife were finalizing the permissions and details of how to reopen the mouth.
What killed wasn't just the river mouth closing — the Tijuana River carried in what Crooks calls nutrients and algae that thrives on those nutrients.
When it rains, the CILA pump station in Tijuana along the concrete channel usually shuts down in order to protect the pumps. As a result, unprocessed sewage and street runoff is sent down the Tijuana River and out to sea. With the river mouth closed this spring, the sewage became food for algae that also consumed all the oxygen. It was the loss of dissolved oxygen that killed, Crooks said.
Through monitors, estuary scientists watched the oxygen levels drop and rise every day.
"The light comes on and photosynthesis starts, and at night it stops," he said. "It's basic science at work."
So, they expected to see highs and lows every day. But on April 8, the dissolved oxygen level dropped and didn't recover.
"This system should be able to withstand a weekend low-oxygen event," Crooks said. "But it didn't come back up and we were literally sitting there [watching monitors and] rooting for the oxygen to come back up."
Imperial Beach city staff and Fish & Wildlife first opened a channel on April 11, and the flooded system drained out furiously, leaving the beach strewn with dead sea life. But the job wasn't over, as the big-wave energy persisted in pushing sand back into the narrow channel. It completely closed up once more, and after that, Crooks said, the crews just continued "nicking away at it to keep it from closing."
Waves versus tides is the basic conflict at work in closing the river mouth, Crooks said.
"Wave energy typically wants to close the system. Tidal action tends to keep it open," he said.